A working machine such as a hydraulic excavator typically has an upper swing structure and a multi-joint front work device composed of a boom and an arm. The multi-joint work device is coupled to the upper swing structure and can be lifted and lowered. A bucket is attached to an end portion of the arm and can be lifted and lowered. Instead of the bucket, the working machine may have a breaker, crasher, grapple or the like attached to the arm to demolish works for structures and wastes, civil engineering construction and the like. The working machine of this type typically has a single front work device. In recent years, however, a working machine (dual arm working machine) has had two front work devices provided on the left and right sides of a front portion of an upper swing structure, as described in Patent Document 1.    Patent Document 1: JP-A-11-181815